Gypsum plasters are common interior plasters in conventional use. The binder typically employed with gypsum plasters is calcium sulphate .beta.-hemihydrate, which contains calcium sulphate in an amount greater than 70% by weight. Also included in these conventional plasters are aggregates and additives, or, in some cases, a multi-phase gypsum plaster. An example of a multi-phase plaster is a plaster containing calcium sulphate .beta.-hemihydrate, calcium sulphate anhydrite-II and optionally calcium anhydrite-III, with the aggregates and additives being present in an amount of about 10% by weight. The solidification characteristics of interior plasters are adjusted in accordance with the requirements of the practitioner.
Lime/gypsum interior plasters are likewise used by those skilled in the art. The lime/gypsum plasters typically use a calcium sulphate .beta.-hemihydrate binder in an amount of about 40% by weight.
Suitable aggregates include fine sands, which may be added alone or in combination with other light aggregates. Additives used in the art include accelerators, retardants, thickeners and other like materials. The types and amounts of additives incorporated in a plaster will depend upon the specific requirements of the skilled artisan.
The characteristics of an interior plaster most desired by those skilled in the art include good processability, high water demand, high yield and a low strength adapted to the substrate. Additionally, an interior plaster should be hygienically acceptable and have as light a color as possible. Mechanical processability is another desirable property of an interior plaster.
To achieve the above-desired properties using calcium sulphate .alpha.-hemihydrate (calcium sulphate .alpha.-hemihydrate can be formed from a flue gas desulphurization process in accordance with German Patent DE-C 28 19 652) is expensive when compared with the cost of a calcined gypsum. Use of calcium sulphate .alpha.-hemihydrate in interior plasters has also proved unsuitable because of excessively high strengths resulting from the calcium sulphate modification. A disadvantage of high strength interior plasters is their tendency hinder the penetration of nails and to chip. The yields of such interior plasters are unreasonably low.
Flue gas desulphurization of coal-fired--especially black coal-fired--combustion plants yields a very fine residual material consisting primarily of calcium sulphite hemihydrate. An example of flue gas desulphurization in coal-fired combustion plants is the spray absorption process (SAP).